1. Field of the Invention
The present invention is related to data communications. In particular, the present invention is related to a gateway architecture for supporting interworking of a Public Switched Telephone Network (PSTN) Signaling System 7(SS7)-based out-of-band signaling network with a data network such as the Internet or an intranet.
2. Description of the Related Art
Out-of-Band Signaling
Signaling in a telecommunications network involves transmission of information between network elements to provide telecommunications service. Signaling between network elements in a Public Switched Telephone Network (PSTN) includes, for example, transmission of dialed digits, dial tone, call-waiting tone, busy signal, etc.
As illustrated in FIG. 1, out-of-band signaling is signaling that occurs over a separate signaling link (e.g., link 110 and link 111) rather than the trunk (e.g., time division multiplexed (TDM) lines 115) over which transmission of user information, e.g., in the form of voice or data transmissions, occur. In a PSTN, when a call is placed, the dialed digits and other information necessary to establish the call are transmitted out-of-band over a signaling link rather than the trunk circuit 115 over which the actual call is conducted. There are several advantages to transmitting signaling out-of-band, including higher voice or data throughput on the trunk circuit, the ability to transmit signaling information at any time, not just the beginning and end of a call, and the ability to send signaling to network elements regardless of whether there is an associated trunk connection.
Out-of-Band Signaling Using SS7
With reference to FIG. 1, Signaling System 7 (SS7) is a standard defined by the International Telecommunications Union (ITU) Telecommunications Standardization Sector (ITU-T). The standard defines the procedures and protocol that supports out-of-band signaling between telecommunications network elements to effect call set up (either wireline or wireless communications), routing and control, using a Common Channel Signaling (CCS) network. Variations under the standard are promulgated by the American National Standards Institute (ANSI), Bellcore, and the European Telecommunications Standards Institute (ETSI). A PSTN transmits signaling information, e.g., call setup, etc., over dedicated out-of-band signaling links, typically 56 or 64 kilobits per second (kpbs) bidirectional links), rather than transmitting the control information in-band over voice channels.
An SS7 signaling network 105, that is, an out-of-band signaling network that operates in accordance with the SS7 procedures and protocol, has three fundamental components. The components are signaling, or service, switching points(SSPs), signal transfer points (STPs), and signal, or service, control points (SCPs). SSPs are PSTN switches, also referred to as Central Office (CO) switches, such as PSTN switches 101 and 102, that are controlled by SS7 software to originate, terminate and switch calls via the SS7 out-of-band signaling network, hereinafter referred to as the SS7 signaling network or SS7 network. STPs, such as STP 103 within the SS7 network 105, route signaling information throughout the SS7 network to the appropriate destination network element. Service Control Points (SCPs) such as SCP 120 are essentially databases accessed in the SS7 network to, for example, provide information for call processing.
Signaling points are uniquely identified in the SS7 network by numeric point codes that are included in the signaling messages transmitted between signaling points to uniquely identify the source and destination signaling points for the messages. Routing tables associate a point code with a path through the SS7 network so that a source signaling point can transmit a message to a destination signaling point in the SS7 network. An STP is a packet switch that routes incoming messages it receives to a signaling link according to routing information contained in the SS7 messages.
SS7 Signaling Links
Again with reference to FIG. 1, SS7 signaling links is are characterized differently according to their use. The SS7 protocol defines A, B, C, D, E and F links. A links 110, 111 and 112(“access links”) interconnect STP 103 to either an SSP (PSTNs 101 or 102) or an SCP, e.g., SCP 120. A links transfer signaling information to or from end signaling points in an SS7 network. C links (“cross links”) interconnect a mated pair of STPs for reliability. B and D links (“bridge and diagonal links”) interconnect two mated pairs of STPs to transmit signaling information between STPs in the SS7 network. A call placed between SSPs 101 and 102 involves transmitting call setup information between SSPs 101 and 102 via SS7 network 105, specfically, via A link 110, STP 103 and A link 111. Once the call is established, voice, data, etc., is transmitted in a time division multiplexed manner over communications medium 115, i.e., trunk circuit 115. Upon completion of the transfer of user information between SSP 101 and 102, call tear down information is transmitted out-of-band over the SS7 signaling network in the same manner as the call set up information.
SS7 Protocol Layers
SS7 is a layered protocol that provides for the exchange of signaling information in support of, and to maintain, a telecommunications network. A physical layer referred to as Message Transfer Part (MTP) Level 1 defines the electrical characteristics of the signaling links of the SS7 network. The signaling links typically utilize digital channels (DS0s) capable of transmitting a digital signal at 56,000 to 64,000 bits per second, or even T1 or E1 links.
A series of higher layer protocols known as the Message Transfer Part (MTP) Levels 2 and 3, collectively referred to as the Message Transfer Part, are essentially equivalent to layers 2 and 3 of the International Standards Organization (ISO) Open Systems Interconnection (OSI) 7 layer conceptual model. Thus, MTP Level 2 provides link layer functionality, such as error checking, flow control, etc., and MTP Level 3 provides network layer functionality, such as routing and congestion control. The MTP Levels 1, 2 and 3 provide a service to the upper layer SS7 protocols, namely, ISUP, TCAP, SCCP, and OMAP protocols, briefly described below.
Integrated Services Digital Network (ISDN) User Part (ISUP) controls the messages for the set-up and tear down of trunk circuits that carry voice or data between and originating, or source, and terminating, or destination, PSTN Central Offices (CO), which are SSPs in the SS7 network.
Signaling Connection Control Part (SCCP) provides end to end routing of TCAP messages and global title translation. A second function of SCCP is global title translation, which provides incremental routing to free source signaling points from maintaining routing information otherwise necessary to reach every destination signaling point in the SS7 network. The source signaling point can query a STP for routing information and request global title translation. The STP decodes the request and routes messages accordingly on behalf of the source signaling point. Global title translation is utilized, for example, in translating a logical address, such as an 800 number dialed by a user, to an actual address of the corresponding 800 number translation application in the signaling network, i.e., the point code of the SCP and the subsystem number for the 800 number translation application.
Transaction Capabilities Application Part (TCAP) supports the exchange of non-trunk circuit related information between signaling points using a SCCP connectionless service, such as 800 service messages and calling card verification. Finally, Operations, Maintenance, and Administration Part (OMAP) supports the additional signaling network management functions such as testing the STP routing tables for loops within the signaling network.
Integration of Telecom and Datacom Networks
In summary, SS7 based telecommunications networks are in operation throughout the world linking the telecommunications switching infrastructure to support many functions, including basic call control, for which the SS7 based networks provide essential functions, and call supplementary services such as number translation and calling card validation. However, presently, data communication networks lack access to the SS7 network, and, as a result, do not benefit from the infrastructure which might otherwise provide for integrating voice and data services, and increasing data communications bandwidth for user data by moving overhead, control, and signaling traffic to the out-of-band signaling network.
What is needed, therefore, is a gateway to the SS7 network providing for the integration of telecommunications networks and a data network, such as the Internet or an intranet, that will allow users to operate in an environment providing for both voice and data services. By accessing the telecommunications network using SS7, data network elements in the Internet or intranet fit cleanly into the telecommunications network infrastructure as peer SSPs and SCPs and can exchange signaling information with telecommunications network elements for routing and treatment of connections.